The invention relates to a display device comprising a first substrate provided with row electrodes and a second substrate provided with column electrodes, in which overlapping parts of row and column electrodes with an interpositioned layer of electro-optical material define pixels, said electro-optical layer comprising a chiral-nematic liquid crystal material which is capable of assuming a plurality of states, of which at least a focal-conic state and a planar state are stable in the absence of an electric field, further comprising drive means for driving the row electrodes with selection signals and for driving the column electrodes with data signals in conformity with an image to be displayed.
More in general, the invention relates to a display device in which an electro-optical layer is switchable between a plurality of (long-lasting) stable states. A display device based on two (or more) stable states may be used in various applications, for example, when information written once should be maintained for a longer period of time (electronic newspapers, telephony, smart cards, electronic price tags, personal digital assistants, billboards, etc.).
A pixel in such a display device, based on chiral-nematic liquid crystal material has a plurality of stable states, namely a light-transmissive state, which corresponds to the focal-conic state of a layer of liquid crystal material, and a reflecting state which corresponds to the planar state of the layer of liquid crystal material. The color (wavelength) of the reflected light is dependent on the pitch of the liquid crystal material, i.e. the distance through which the director (the average orientation of the molecules in a layer) makes a twist of 360 degrees. In the absence of an electric field, both states are stable for a long period of time. In the light-transmissive states, light of said color is passed to a larger or smaller degree, dependent on the texture (ratio between parts of a pixel in the planar and the focal-conic states, respectively). Moreover, such a display device may also have the so-called homeotropic state; at a high voltage, all molecules (directors) direct themselves to the fields. Incident light then passes through the liquid crystal material in an unhindered way. When used without polarizers, the color in the homeotropic state of a reflective display device is determined by the background color, for example, an absorbing layer. The display device is usually only brought to this state to reach one of the two stable states. Dependent on the frequency used and on the voltage of the switching pulses, a pixel changes to the focal-conic or the planar state.
The selection time (addressing time) for writing the different states is usually rather long. Without special measures, it is 20 to 30 msec, which is too long for use in, for example, an electronic newspaper.
The article xe2x80x9cDynamic Drive for Bistable Cholesteric Displays; A Rapid Addressing Schemexe2x80x9d, SID 95 Digest, page 347 describes how the addressing time which is necessary for reaching the different states can be reduced by means of a special drive mode, using a preparation phase and an evolution phase.
It is, inter alia, an object of the present invention to reduce the selection period. To this end, a display device according to the invention is characterized in that, in the operating state, the drive means sequentially provide groups of p row electrodes (p greater than 1) with mutually orthogonal signals during a selection period.
The use of orthogonal signals is known per se for driving (super)twisted nematic display devices so as to inhibit a phenomenon which is known as frame response. In contrast to the conventional single line addressing, a number of rows is selected simultaneously. This requires a special treatment of incoming signals which must be processed mathematically so as to determine the correct signals for the column electrodes. Said phenomenon of frame response occurs when the frame time becomes too long in proportion to the response time of the liquid crystal material. The transmission of a pixel is then no longer determined by the effective voltage value in a plurality of successive selections, but follows the presented voltage pattern to a greater or lesser degree. In the case of orthogonal drive, the drive signals are adapted in such a way that a pixel is driven several times per frame period. The transmission is then again determined by said effective voltage value in a plurality of successive selections. Notably when used in the above-mentioned applications (electronic newspapers, telephony, smart cards and electronic price tags) of chiral-nematic liquid crystal material, in which the drive voltage is removed after information has been written once, such a problem does not occur in the absence of successive selections.
The invention is based on the recognition that the selection period should be sufficiently long, on the one hand, so that the liquid crystal (the pixel) reacts to the effective voltage value of the presented signals, whereas, on the other hand, a plurality of rows (p) can be simultaneously driven with orthogonal signals within the selection period, while a column signal is determined by the desired state of the pixels and the corresponding orthogonal signals on the rows. In the simultaneously driven rows, sufficient energy is presented to cause the pixels to switch. Consequently, the display device is written faster by a factor of p. The p rows may be spread on the surface of the display device but preferably form a group of consecutive rows. The optimum value for p appears to be dependent on the electro-optical characteristic of the pixels, such that             p      opt        =          16.      ⁢                        V          pf          2                ⁡                  [                                                                      1                  2                                ⁢                                  xe2x80x83                                ⁢                                  (                                                            V                      on                      2                                        +                                          V                      off                      2                                                        )                                            -                              V                pf                2                                                                    (                                                      V                    on                    2                                    -                                      V                    off                    2                                                  )                            2                                ]                      ,
in which Von is the voltage across a pixel in the reflection (transmission)/voltage characteristic curve required for the transition to a planar state via the homeotropic state, Voff is the voltage across a pixel in the reflection (transmission)/voltage characteristic curve for the transition to the focal-conic state, and Vpf is the voltage across a pixel in the reflection (transmission)/voltage characteristic curve for the transition from the planar state to the focal-conic state.
In principle, Vpf, Von and Voff are related to reaching a certain reflection (transmission), for example 99%, 99% and 1% of the maximum reflection (or, for example 95%, 95% and 5%). In practice, notably Von and Voff are often also determined by the adjustment of the drive circuit (driver IC).
Moreover, the reflection (transmission)/voltage characteristic also depends on the history. In some cases, the state reached after selection depends on the initial situation and may be different for an initial situation in which the pixel at a voltage of 0 volt is in the focal-conic state, as compared with an initial situation in which the pixel at a voltage of 0 volt is in the planar state. This is not a problem for on-off switching (for example, alphanumerical) displays but is a problem in the case of fast changes in the image in which grey scales are also to be displayed. To provide this facility, a preferred embodiment of a display device according to the invention is characterized in that the drive means comprise means for bringing, prior to a selection period, the liquid crystal material in groups of p rows of pixels to an (unambiguously) defined state in the operating state. This defined state is preferably the homeotropic state, but the focal-conic state is alternatively possible, while even a state associated with a given texture (grey value) is feasible.
For the orthogonal functions, for example, Walsh functions are chosen, but other functions are alternatively possible such as, for example, Haar functions, Rademacher functions or Slant functions. To prevent a DC voltage from being built up when driving the same kind of information for a long period of time (for example, a title of a document at the top of a page whose contents change, or the word xe2x80x9cpagexe2x80x9d at the bottom of a page of an electronic newspaper), the voltage integral of the selection voltages in a selection period is preferably zero.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.